Numerous braking systems already exist that are fitted with a hydraulic circuit that is used when conditions are normal, and with an emergency hydraulic circuit powered by a different source of hydraulic pressure that takes over in the event of a breakdown.
In a first example, a mechanical-hydraulic braking system is provided in which the two sources of hydraulic pressure are applied to an upstream selector that is provided to perform automatic switching. In this context, reference may be made to the following documents: U.S. Pat. No. 5,024,491 and U.S. Pat. No. 3,926,479. Downstream from the automatic selector, there are two separate hydraulic circuits, with a normal circuit that connects a first double metering valve to the wheel brakes via respective servo-valves associated with each brake and under electrical control of the control unit, and with an emergency circuit connecting a second double metering valve to the wheel brakes via associated shuttle valves. The two metering valves are actuated directly by the brake pedals via associated rodding. The presence of shuttle valves provided on the emergency circuit may differ depending on the type of brake used. With two-cavity brakes, each servo-valve of the normal circuit feeds a half-brake of the corresponding wheel, and each servo-valve of the emergency feeds the other half-brake of the associated pair of wheels (left or right), whereas with single cavity brakes the emergency circuit connects the corresponding double metering valve to pairs of wheels (left or right) via servo-valves (controlled by the control unit) associated with respective pairs of wheels, and the shuttle valves of the emergency circuit may be integrated in the brakes. In this context, reference may be made to the document EP-A-0 443 213 where an organization of that type for a pair of wheels is described, and in which control is specially designed to increase the lifetime of brakes having carbon disks.
The drawback of such a system lies in its inflexibility in use, due mainly to the automatic upstream selector, making it impossible to take maximum advantage of the redundancy provided in the circuit. This selector always give precedence to the normal circuit over the emergency circuit because it compares the hydraulic pressure in the normal circuit with a predetermined percentage of nominal pressure. Consequently, a breakdown occurring downstream from the selector is not "seen" by the automatic selector, so the emergency circuit is not engaged under such circumstances. Further, since the pilot cannot choose between the normal circuit and the emergency circuit, the emergency circuit cannot be used to obtain extra braking power, such that for parking purposes it is necessary to provide a third hydraulic circuit connected to the pressure source of the emergency circuit and connecting a mechanically controlled metering valve to the shuttle valves of the emergency circuit. Furthermore, in the event of a minor breakdown on the normal circuit (e.g. misfunction of a servo-valve that also performs the anti-skid function), the computer in the control unit issues a warning signal, but the only option available to the pilot for countering the breakdown is to make use of the parking brake.
A second example is known in which the normal circuit and the emergency circuit are organized similarly to the example described above, except insofar as both circuits terminate at shuttle valves associated with each wheel brake. The controls are organized differently in that the brake pedals are connected to the two double metering valves not by rodding, but by master cylinders that control pressure: brake control is thus of the hydro-hydraulic type. That system is easier to install than the preceding system, and it avoids the need to balance tensions in cabling. Furthermore, electrical control enables the pilot to act directly on the automatic selector, enabling the emergency circuit to be used whenever desired. However, that action can only be taken under the direct control of the pilot, and is therefore necessarily subject to error in that it may be taken too late and/or under conditions that are unsuitable, and this is all the more likely in that the pilot may well be coping simultaneously with other problems (e.g. a fire in one of the engines): the risk of human error therefore penalizes that type of system.
A third example is known that constitutes a combination, having an emergency circuit that is controlled hydro-hydraulically by means of master cylinders (as in the second example), but in association with a normal circuit that is controlled electro-hydraulically and that additionally includes a solenoid valve (ON/OFF valve) provided upstream from the automatic selector: if the solenoid valve is off, then the selector switches automatically to the emergency circuit. The emergency circuit is then the only circuit to be fitted with a double metering valve connected to the brake pedals by means of master cylinders. A system of that type is illustrated in document FR-A-2 608 987 (& U.S. Pat. No. 4,834,465) in the name of the Applicant. That system thus has the advantage of being more flexible than the preceding systems since, in the event of a breakdown, the computer of the control unit can switch automatically to the emergency circuit, thereby taking better advantage of the redundancy provided in the circuits. Nevertheless, the above-mentioned drawbacks inherent to the presence of the upstream selector remain. In addition, if the additional solenoid valve should break down with the valve jammed in the on position, then only the normal circuit can be used and no advantage can be taken of the emergency circuit. Finally, as in the first example, a third circuit is provided for parking, with an electrically-controlled metering valve feeding pressure to the shuttle valves of the emergency circuit.
It should also be specified that the drawbacks mentioned for the three above examples and inherent to upstream selection between the two main circuits of the braking system are aggravated with increasing number of wheels in the landing gear, constituting an even greater handicap in the systems fitted to aircraft of large capacity (often fitted with two wing undercarriage units and two fuselage undercarriage units, giving a total of sixteen wheels).
Mention may also be made of a fourth example using electro-hydraulic control and based on simultaneously activating the normal circuit and the emergency circuit, both of which are then organized identically, with a third circuit still being provided for parking, as in the first and third examples. Such permanent coupling gives rise to a certain amount of inflexibility, particularly with respect to the anti-skid function provided by the servo-valves associated with each brake (the anti-skid order depends on two wheels).
To finish off the state of the art, the document WO-A-91 11352 may also be mentioned which describes electro-hydraulic braking control in which the hydraulic pressure source is doubled-up: the emergency (secondary) circuit takes over automatically for braking purposes if the pressure in the normal (primary) circuit drops beneath a predetermined threshold below the pressure that exists in the emergency circuit. That system is highly redundant since it is necessary to provide two servo-valves per brake (one for the primary circuit and one of the secondary circuit).
Finally, mention may be made of document FR-A-2 038 801 which describes an architecture comprising two independent circuits without selection being possible, and document DE-C-1 118 020 which illustrates a mechanical-hydraulic type system in which the hydraulic source is brought into play automatically without any selection being possible, and it is applied simultaneously to all of the associated actuators (it should be observed that said system does not include any anti-skid valves).